The present invention relates to the field of coal-fired furnaces. It relates particularly to a furnace that can use coal for ignition, warm-up, and low-load stabilization.
It is sometimes desirable to use coal rather than gas or oil in electrical generating facilities. In those situations, the utility will naturally have a coal-fired unit built rather than an oil-fired unit. However, even in coal-fired units, substantial quantities of gas or oil are often used. In a typical coal-fired unit, coal to be burned in the furnace is dried and pulverized in a coal mill and delivered directly from the coal mill to the load-carrying coal nozzles in the furnace. Operation of the coal mills requires that heated air be supplied to the mills for drying and conveying the coal. This air is supplied by a forced-draft fan that forces the air through an air preheater, a device that uses the hot products of combustion in the furnace to preheat the air. This preheated primary air, the air used for drying and conveying coal, is delivered with the coal to the coal nozzles and used to support combustion. The primary air is typically not sufficient in quantity to support combustion of all the coal, so secondary air is brought directly from the air preheater to the furnace to supply the rest of the air needed for combustion. The coal thus supplied with air is caused to burn due to ignition energy from the primary air, the secondary air, the heat in the coal itself, radiation and conduction from flame in the furnace, and radiation from furnace walls.
It is to be noted that almost all of these combustion energy sources presuppose that the furnace has already been operating, and, in the large furnaces used in power generation, it presupposes that the furnace has been operating for a fairly long time. Accordingly, in order to cause and sustain combustion of the coal, it is necessary to use an auxiliary fuel for warming up the furnace walls, for providing ignition flame, and for warming up the air preheater. This is usually the function of oil- or gas-fired ignitors and warm-up guns.
In a typical installation, a relatively high-capacity oil burner is started by an ignitor, and this starts the process or warming up the furnace walls and the heat-exchange surfaces of the air preheater. This can take some time, and the use of 70,000 gallons of oil in a 900-megawatt unit for one startup alone is not uncommon. In addition, there is considerable capital expense involved in providing the hardware that is used for supplying oil. Once the furnace has been brought up to temperature, the coal nozzles are ignited by oil- or gas-fired ignitors or by the warm-up guns themselves.
The use of auiliary fuel is not necessarily over when the coal nozzles have started to supply coal. At higher boiler loads--that is, when the amount of coal supplied by the nozzles is great--the furnace can typically maintain stable combustion of the pulverized coal. However, when the load goes down and the coal supply is thereby decreased, the stability of the pulverized coal flame is also decreased, and it is therefore common practice to use the ignitors or warm-up guns to maintain flame in the furnace, thus avoiding the accumulation of unburned coal dust in the furnace and the associated danger of explosion.
All of these functions of the oil- or gas-fired burners rely on the greater ease of ignition of these fuels; less heat is required, from whatever source, to liberate the volatiles and thereby initiate or sustain combustion. Conversely, the greater difficulty encountered in igniting coal is the reason why it has typically not been used for the ignition, warm-up and low-load-stabilization functions. An incidental advantage of oil and gas that also contributes to the greater desirability of their use for these functions is that it is possible to supply them in relatively small pipes, thereby keeping their contribution to the congestion in the fuel-nozzle area to a minimum. The usual method of supplying coal to nozzles has required rather large piping, and the addition of more large-size piping would not be welcome in the area immediately behind the fuel nozzles.